As a spark plug used as an ignition means in an internal combustion engine, such as an automotive engine, there is one in which a center electrode and a ground electrode are made to axially face each other to form a spark discharge gap. Such a spark plug causes discharge at the spark discharge gap and ignites air-fuel mixture in a combustion chamber by the discharge.
Here, in the combustion chamber, airflow of the air-fuel mixture, for example, such as swirl flow or tumble flow, is formed and the airflow moderately flows also at the spark discharge gap to thereby ensure ignitability.
However, depending on a mounting posture of the spark plug to the internal combustion engine, a part of the ground electrode joined to a tip part of a housing may be arranged on an upstream side of the spark discharge gap in the airflow. In this case, the airflow in the combustion chamber is blocked by the ground electrode, and the airflow near the spark discharge gap may stagnate. As a result, ignitability of the spark plug may degrade. That is, there may occur a problem that ignitability of the spark plug varies depending on the mounting posture to the internal combustion engine. Particularly, in recent years, although internal combustion engines using lean burn are used frequently, in such an internal combustion engine, combustion stability may degrade depending on the mounting posture of the spark plug.
In addition, it is difficult to control the mounting posture of the spark plug to the internal combustion engine, that is, the position of the ground electrode in a circumferential direction unless special measures are taken. This is because the mounting posture varies due to the formation state of a mounting screw in the housing or the tightening degree of the spark plug at the time of mounting work to the internal combustion engine. Note that, it may be considered to take such a special measure as to limit the relation between the mounting screw and the joining position of the ground electrode in the circumferential direction of the spark plug to a specific positional relation and also to limit a female screw on an engine head side to a predetermined direction in the circumferential direction. However, in this case, there is a problem that this leads to increase in manufacturing effort and manufacturing cost of the spark plug and the engine head.
Therefore, to suppress airflow from being blocked by the ground electrode, a configuration in which drilling is performed on the ground electrode and a configuration in which the ground electrode is joined to the housing by a plurality of thin plate members are disclosed (Patent Literature 1).